i5-2500k is a much better choice, especially if you want your system to last longer. When new processors come out the previous generation will always be slower, however with overclocking you can match the current speed of the new processors.
For overclocking you'll want some aftermarket cooling and a P67(or)Z68 board.
What are your uses? i5-2500k is good for anything CPU intensive. i5-2400 will be better for a machine you want limited power usage/heat in but don't need super CPU clocks.
get the 2500k its better and its unlocked. so you can overclock it and you wont need to upgrade as soon. the 2400 is a waste of money just pay the extra 30 bucks or whatever and get something thats not a teeze